The advent of portable computers has created intense demand for display devices which are lightweight, compact and power efficient. Since the space available for the display function of these devices precludes the use of a conventional cathode ray tube (CRT), there has been significant interest in efforts to provide satisfactory flat panel displays having comparable or even superior display characteristics, e.g., brightness, resolution, versatility in display, power consumption, etc. These efforts, while producing flat panel displays that are useful for some applications, have not produced a display that can compare to a conventional CRT.
Currently, liquid crystal displays are used almost universally for lap top and notebook computers. In comparison to a CRT, these displays have limited brightness, only a limited range of viewing angles, and, in color versions, they consume power at rates which are incompatible with extended battery operation. In addition, color liquid crystal display screens tend to be far more costly than CRT's which have an equal screen size.
As a result of the drawbacks of liquid crystal display technology, field emission display technology has been receiving increasing attention by industry. Flat panel displays utilizing such technology employ a matrix-addressable array of pointed, thin-film, cold field emission cathodes in combination with an anode comprising a phosphor-luminescent screen. The phenomenon of field emission was discovered in the 1950's, and extensive research by many individuals, such as Charles A. Spindt of SRI International, has improved the technology to the extent that its prospects for use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, full-color flat displays is promising.
Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued Aug. 28, 1973, to C. A. Spindt et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued Jul. 10, 1990 to Michel Borel et al.; U.S. Pat. No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued Mar. 16, 1993 to Robed Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued Jul. 6, 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
The Clerc ('820) patent discloses a trichromatic field emission flat panel display having a first substrate on which are arranged a matrix of conductors. In one direction of the matrix, conductive columns comprising the cathode electrode support the microtips. In the other direction, above the column conductors, are perforated conductive rows comprising the grid electrode. The row and column conductors are separated by an insulating layer having apertures permitting the passage of the microtips, each intersection of a row and column corresponding to a pixel.
On a second substrate facing the first, the display has regularly spaced, parallel conductive stripes comprising the anode electrode. These stripes are alternately covered by a first material luminescing in the red, a second material luminescing in the green, and a third material luminescing in the blue, the conductive stripes covered by the same luminescent material being electrically interconnected.
The Clerc patent discloses a process for addressing a trichromatic field emission flat panel display. The process consists of successively raising each set of interconnected anode stripes periodically to a first potential which is sufficient to attract the electrons emitted by the microtips of the cathode conductors corresponding to the pixels which are to be illuminated or "switched on" in the color of the selected anode stripes. Those anode stripes which are not being selected are set to a potential such that the electrons emitted by the microtips are repelled or have an energy level below the threshold cathodoluminescence energy level of the luminescent materials covering those unselected anode stripes.
A shortcoming of field emission displays of the current technology is the low emission intensity of the low voltage phosphors typically used as the luminescent material on the display screen. The low emission intensity of the phosphor has several origins, one of which is the low acceleration voltage used to excite the free electrons toward the anode. Currently, this acceleration voltage is limited by the potential which can be placed between adjacent transparent stripe anode conductors underlaying the phosphor stripes, typically about 300-500 volts. It is known that significantly improved performance and image brightness would be provided by increasing the anode potential to about 1000 volts. However, as the acceleration voltage is increased, the leakage current between the conductive anode stripes increases, and it is possible that high voltage breakdown can occur.
When a high voltage breakdown occurs, there is arcing between anode stripes as current flows across the anode surface from the anode stripe which is at a high potential to an adjacent anode stripe which is at a low potential. The arcing may also occur through the vacuum space between the anode stripes; however, at the vacuum levels commonly used in the FED it is unlikely that the breakdown would occur through the vacuum before occurring across the anode surface. During the high voltage breakdown, the user may see a dimming of the display image where the current is leaving the high potential anode stripe. In addition, the user may simultaneously see a color bleed as an anode stripe which was at low potential receives current, and as a result, the phosphors at that location luminesce.
Factors contributing to the breakdown voltage between adjacent anode stripes include anode stripe geometry, surface conditions, the applied electric field, and transport time. The anode stripe geometry affects the breakdown voltage level because any sharp edges located on the anode stripe create an enhanced electric field during display operation and therefore lowers the voltage level at which breakdown will occur. The surface condition of the anode plate between the anode stripes affects the breakdown voltage level because contaminants present on the surface may encourage the flow of electrons between the anode stripes. In addition, the material composition of the surface affects the breakdown voltage level due to the inherent properties of water absorption, outgasing, and charge properties. The applied electric field affects the breakdown voltage because the leakage current is directly proportional to the potential applied to the anode stripe. Also, the higher the potential on the anode stripe the higher the chances are for a voltage breakdown below operating voltage. Transport time is the time it takes for the electrons to travel along the surface between the anode stripes. Therefore, if the anode stripe is not charged for a time long enough for the current to flow between anode stripes a high voltage breakdown will not occur. The mechanisms which affect high voltage breakdown are discussed in more detail in IEEE Trans. Electr. Insul., Sudarshan, T. S., Cross, J. D., Srivastava, K. D., "Prebreakdown Processes Associated With Surface Flashover of Solid Insulators in Vacuum," pp. 200-208, Vol. E1-12, No. 3 June, 1977, and IEEE Trans. Electr. Insul., Tourreil, C. H., Srivastava, K. D., "Mechanism of Surface Charging of High-Voltage Insulators in Vacuum," pp.17-21, Vol. E1-8, No. 1, March 1973, both incorporated herein by reference.
Increasing the anode potential to increase luminance has many benefits. For example, increasing the luminance permits the display image to be clearly visible in environments of bright ambient light, such as outdoor sunlight. An increased display luminance also accommodates FED overhead projector applications. As described above, increasing the anode potential to realize these benefits increases the likelihood of a high voltage breakdown. Therefore, the anode stripes may need to be spaced farther apart in high voltage applications to protect the apparatus against the occurrence of a high voltage breakdown.
Unfortunately, spacing the anode stripe conductors further apart to accommodate the high luminance applications decreases the image resolution. Decreasing the image resolution makes the display image less defined and therefore, the product will be less desirable to the user. Furthermore, future applications will demand higher resolutions and therefore closer spacing of the anode stripes. For example, while the most common resolution used today is a VGA standard of 640 pixels by 480 pixels for a 10" diagonal display, some applications exist which require the SVGA standard of 800 pixels by 600 pixels, or even require the XGA standard of 1240 pixels by 1080 pixels for the same display size.
In view of the above, it is clear that there exists a need for an improved method of fabricating the anode plate of a field emission flat panel display device such that the anode plate facilitates an increased acceleration voltage to thereby provide higher luminance and greater display image resolution.